Method for doping semiconductor layer, method for manufacturing thin film semiconductor device, and thin film semiconductor device

ABSTRACT

A low concentration impurity diffusion region is formed with good controllability even in case of using a low heat resistant substrate. When doping a semiconductor layer, after forming the semiconductor layer on the substrate, the amount of the dopant ion adsorbed on a surface of the semiconductor layer is controlled by introducing hydrogen gas at the time of plasma irradiation and activating the adsorbed dopant ion in the semiconductor layer by an excimer laser.

RELATED APPLICATION DATA

This application is a divisional of co-pending application Ser. No.10/433,849 filed Dec. 15, 2003, incorporated herein to the extentpermitted by law. The present and foregoing applications claim priorityto Japanese Application No. P2000-373052 filed on Dec. 7, 2000, and PCTAppliation No. PCT/JP01/10726 filed Dec. 7, 2001.

BACKGROUND OF THE INVENTION

The present invention relates to a method for doping a semiconductorlayer, a method for manufacturing a thin film semiconductor device,method for controlling resistance of a semiconductor layer, and a thinfilm semiconductor device, and more particularly, a doping method usinga crystallized semiconductor layer by excimer laser anneal, a method formanufacturing a thin film semiconductor device such as a thin filmtransistor, a thin film semiconductor device in which a semiconductorlayer made of such as polycrystalline silicon is used as a channel.

With progress of an advanced information age, the importance ofinput/output devices is increasing rapidly and the devices are in demandto include advanced and sophisticated features. Furthermore, the spreadof personal digital assistant machines is remarkable in recent years,and consequently, the technology of producing TFT on a plastic substratewith more excellent weight saving, flexibility, and non-destructivityrather compared with a glass substrates is desired. In such a situation,research and development of active matrix liquid crystal display devices(AM-LCD) using a thin film transistor (TFT) and contact image sensors(CIS) and the like are actively done.

The thin film transistors, in which a semiconductor film made of siliconis used as a channel, can be classified by a material used in order toform a carrier-transporting layer (active layer), that is, asemiconductor film made of amorphous silicon (a-Si) and a semiconductorfilm made of polycrystalline silicon having a crystal phase. Polysilicon(poly-Si) or microcrystal silicon (μc-Si) is mainly known as thepolycrystalline silicon.

Semiconductors made of the polycrystalline silicon such as polysilicon(poly-Si) or microcrystal silicon (μc-Si) are characterized by thecarrier mobility from about 10 to 100 times as high as that ofsemiconductors made of amorphous silicon, and have very excellentcharacteristics as a composition material of switching elements.Moreover, the thin film transistors using the polycrystalline siliconfor the active layer allow high-speed operation, and therefore aregetting most of the attention as the switching elements constitutingvarious logical circuits (for example, a domino logic circuit, a CMOS(Complementary Metal Oxide Semiconductor) transmission gate circuit),multiplexers using these circuits, EPROM (Erasable and Programmable ReadOnly Memory), EEPROM (Electrically Erasable and Programmable Read OnlyMemory), CCD (Charge Coupled Device), RAM (Random Access Memory), drivecircuits of displays such as a liquid crystal display and anelectroluminescent display, and the like in recent years. Moreover,recently, remarkable are active matrix liquid crystal displays employingthe thin film transistor (TFT), using such polysilicon for a channelsemiconductor film, as the switching element and as a peripheral drivecircuit. This is because the constitution of a thin film transistorarray, making use of a polysilicon semiconductor film which can beformed at a low temperature on a cheap amorphous glass substrate, mayallow to implement reflective panel displays or wide, high-finesse,high-definition and cheap panel displays (for example, a flat typetelevision).

On the other hand, when using poly-Si TFT in switching elements forpixel selection of the liquid crystal display or the like, the offcurrent is high and display quality is low, which is a problem. Inconventional MOS transistors using single crystal silicon, when a gatereverse bias is applied to a gate, a leakage current does not increase,since the channel become a opposite polarity with a source or a drain, adepletion layer is formed and enough pressure-proofing and rectificationproperty is shown. However, with the poly-Si TFT, a problem arises thata high leakage current occurs since current flows through the grainboundary of crystalline particles composing the semiconductor film orthrough the defects of the particles themselves. Furthermore, since theMOS transistors are not used under very high gate reverse bias, theleakage current has not become a problem. However, in the poly-Si TFTusing for the active matrix liquid crystal displays, for example, theleakage current poses a big problem since it is used under the reversebias of about 10 V or more. Such a problem is especially important forthe thin film transistor for pixel selection of the liquid crystaldisplays in which the poly-Si is used.

In order to reduce the leakage current, it is effective to relax theelectric field in the drain edge, and it has been known that LDD(Lightly Doped Drain) structure is effective (General Conference of TheInstitute of Electronics and Communication Engineers, 2-20, pp. 271,1978). The structure forms the region which activated the impuritiesunder a low dose such as 1×10¹⁴/cm² or less in the edge of the drainregion to relaxes the electric field in the edge of the drain region.

The thin film transistor having the LDD structure is formed, forexample, by the following processes so far. First, an amorphous siliconcontaining hydrogen (a-Si:H) film is formed on a glass substrate, and isdehydrogenated by the lamp anneal. Then, a polysilicon (poly-Si)semiconductor film is formed by crystallizing the amorphous silicon filmby laser irradiation. Then, a gate insulating film and a gate electrodeare formed, and heavy doping of impurity ion is performed by using thegate electrode as a mask where the gate electrode has already beenpatterned to cover a channel region and an LDD region. Subsequently, thegate electrode is patterned again to cover only the channel region.Further, light doping of impurity ion is performed by using there-patterned gate electrode as a mask. Consequently, source and drainregions having the LDD structure is formed. Such processes have beendisclosed in Japanese Unexamined Patent Application No. 2000-228526.

When forming the thin film transistor having the LDD structure by such amethod, there is a problem of the difference or the variation in lengthsof the LDDs on both sides of the channel region (thicknesses of the LDDregions between the channel regions and contact regions) due todeviation of the mask during patterning of the gate electrode, and thelike. This causes other problems that the characteristics of the thinfilm transistor vary and the productivity of the thin film transistordecrease. Moreover, the LDD lengths cannot be set to about 2 μm or lessin order to secure a mask alignment margin. For this reason, theresistance of the LDD regions becomes high, and the carrier mobilitydecreases, which is a problem. Therefore, it is important to develop theself-alignment type process where the controllability of the LDD lengthsis enough at a low dose such as 1×10¹⁴/cm² or less.

By the way, as for the poly-Si TFT, the highest process temperaturereaches about 1000° C. in the manufacturing process. Therefore, silicaglasses or the like having an excellent heat resistance are used as aninsulating substrate for manufacturing the poly-Si TFT. That is, it isdifficult to use a glass substrate with a comparatively low meltingpoint in the manufacturing process. However, for a cost reduction of theliquid crystal displays, the use of the glass plate materials with a lowmelting point is indispensable. Then, in recent years, the developmentof the so-called low temperature process with the highest processtemperature of 600° C. or below is making progress, and the productionof such devices is practically done. Furthermore, recently, using aplastic substrate which is easy to form a larger area under lowertemperature has been also examined. The deformation temperature of theplastic substrate is at most 200° C., even when formed from a heatresistant material. Therefore, when the substrate is formed from theplastic, all processes must be performed on the condition of super lowtemperature as compared with the conventional conditions, that is, at200° C. or below.

With the larger type of liquid crystal display, in the low temperatureprocess for the poly-Si TFT, the ion doping and the plasma doping, whichallow doping impurities into the semiconductor thin film with a largearea with a fine throughput, are used. The ion doping is the method ofionizing an impurity gas and then irradiating the impurity ion all atonce onto the large area semiconductor thin film by acceleratingelectric field without performing a mass separation. The plasma dopingis the method of ionizing an impurity gas and a deposition gassimultaneously, and deposit including the impurity ion on the substratesurface. On the other hand, ion implantation is the method of performingthe mass separation of impurity ion, producing an ion beam of theseparated ion and irradiating the ion beam onto the semiconductor thinfilm. Although the ion doping and the plasma doping are advantageous tothe formation of the larger area type, these processes pose problemsthat the film can contain hydrogen in large quantities which can blowoff and break the film at the time of crystallization by the excimerlaser (ELA: Excimer Laser Anneal), and that it is difficult to performthe lower temperature process using the plastic substrate or the like atthe required temperature for dehydrogenation (400° C.). Moreover, thereis also a problem that these methods are not suitable for theself-alignment type process in principle.

By the way, the Laser-Induced Melting of Predeposited Impurity Doping(LIMPID) attracts attention recently as being a method in which dopingcan be done in a process at 200° C. or below. The LIMPID is the methodof ionizing an impurity gas, adsorbing the impurity ion on the surfaceof the semiconductor thin film, and melting the ion into the film withan excimer laser, and attracts attention not only because the hydrogencannot be entrapped into the film, but also because it is mostappropriate to the self-alignment process as well as to the lowtemperature process (refer to Japanese Unexamined Patent Application No.SHO 61-138131, Japanese Unexamined Patent Application No. SHO 62-002531,Japanese Unexamined Patent Application No. SHO 62-264619, and JapaneseUnexamined Patent Application No. HEI 9-293878).

With the LIMPID, the high dose such as from about 1×10¹⁵ to 1×10¹⁶/cm²of the impurities can be electrically activated in the semiconductorthin film. However, in principle, it is difficult to precisely controlthe dose of 1×10¹⁴/cm² or less of the impurities. Because the high doseof from about 1×10¹⁵ to 1×10¹⁶/cm² of the impurities is activated by theexcimer laser anneal, even when, for example, the impurity ion of anatomic layer are adsorbed on the top of Si surface. Furthermore, sincethe adsorption of the impurity ion of the atomic layer occurs for anextremely short time in the conventional methods, the control at the lowdose is difficult.

FIG. 13 shows a sheet resistance ρ_(s) in a case where the anneal isperformed by use of the excimer laser after adsorbing phosphorus byplasma irradiation. The sheet resistance ρ_(s) is measured by changingthe partial pressure of phosphine (PH₃) by an argon gas as an inert gas.The conditions of the plasma irradiation are as follows: the flow rateof argon gas is 5 to 150 sccm, the flow rate of phosphine and hydrogenis 3 to 10 sccm, the total pressure is 63 Pa (475 mTorr), the substratetemperature is 130° C., RF power is 20 W and the irradiation time is 1minute. The anneal is performed by use of XeCl excimer laser of 308 nmin wavelength, with the energy density of 300 mJ/cm² and the overlapratio is 98%. As seen in FIG. 13, even when the partial pressure ofphosphine is changed, the sheet resistance ρ_(s) changes little and itreveals that controlling the partial pressure cannot control theconcentration of impurities in an impurity diffusion region.

On the other hand, the conventional ion implantation is the mostappropriate to the self-alignment process and enables also the controlat a low dose. Since the substrate temperature generally increases inthe process for the silicon substrate, the method of attaching a coolingplate by the electrostatic chuck of the substrate and radiating heatfrom the back side thereof is taken in the process. However, it isdifficult to apply such a method to the plastic substrate consideringthe thermal conductivity and electrical conductivity of the plasticsubstrate. Moreover, there are other problems that the impurities cannotbe implanted into the semiconductor thin film with the large area all atonce, and that the throughput gets worse in the manufacturing thelarge-sized liquid crystal displays.

The present invention has been achieved in view of the above problems.It is an object of the invention to provide a method for doping asemiconductor layer which can form a lower concentration impuritydiffusion region under excellent controllability even when a low heatresistant substrate is used, a method for manufacturing a thin filmsemiconductor device, a method for controlling resistance of asemiconductor layer, and a thin film semiconductor device.

SUMMARY OF THE INVENTION

A method for doping a semiconductor layer of the invention comprises thesteps of: forming a semiconductor layer on a substrate; adsorbing dopantion on a surface of the semiconductor layer and controlling the amountof the adsorption; and activating the adsorbed dopant ion in thesemiconductor layer.

A method for controlling the amount of the dopant ion adsorbed on thesurface of the semiconductor layer includes, for example, a method forcontrolling the rate of termination in a dangling bond of the materialsforming the semiconductor layer, and a method of selectively removingthe dopant ion adsorbed on the surface of the semiconductor layer byetching. Examples of the method for controlling the rate of thetermination in the dangling bonds are adsorbing the hydrogen ion on theends of the dangling bonds and are changing the substrate temperature onwhich the semiconductor layer is formed. Concretely, a method ofdiluting the dopant ion gas to be adsorbed with hydrogen gas or mixedgas of hydrogen gas and inert gas to have 1% or less of theconcentration of the dopant ion can be used.

According to the method for doping the semiconductor layer of theinvention, the dopant ion is adsorbed on the surface of thesemiconductor layer formed on the substrate. The amount of the dopantion is controlled so that the amount to be activated after introducingthe semiconductor layer is precisely controlled, thereby the lowconcentration impurity diffusion is formed on particularly a low heatresistant substrate with good reproducibility.

Further, a method for manufacturing a thin film semiconductor device anda method for controlling resistance in the semiconductor layer of theinvention comprises the steps of: forming a semiconductor layer on asubstrate; adsorbing dopant ion on a surface of the semiconductor layerand controlling the amount of the adsorption; and activating theadsorbed dopant ion in the semiconductor layer. A thin filmsemiconductor device of the invention is manufactured with the abovemanufacturing method.

Other and further objects, features and advantages of the invention willappear more fully from the following description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A to 1C are views illustrating the steps in a method for doping asemiconductor layer according to an embodiment of the invention. FIG. 1Ais a view illustrating the state of a surface of a silicon layer beforeion adsorption, FIG. 1B is a view illustrating the state of the surfaceof the silicon layer in which phosphorus ion and hydrogen ion isadsorbed, and FIG. 1C is a view illustrating the step of activation byirradiation of excimer laser.

FIG. 2 is a characteristic view illustrating the relation of the mixtureratio of diluted gas and the sheet resistance according to an example ofthe invention.

FIG. 3 is a characteristic view illustrating the relation of the mixtureratio of diluted gas and the concentration of phosphorus according tothe example of the invention.

FIG. 4 is a characteristic view illustrating the relation of the sheetresistance and the plasma irradiation time according to the example ofthe invention.

FIG. 5 is a characteristic view illustrating the relation of thesubstrate temperature and the sheet resistance according to the exampleof the invention.

FIG. 6 is a characteristic view illustrating the relation of thetreatment time of hydrogen plasma and the sheet resistance according tothe example of the invention.

FIG. 7 is a view illustrating the distribution of film thickness insidea wafer after CVD deposition and the sheet resistance evaluation points.

FIG. 8 is a view illustrating the in-plane distribution of the sheetresistance by LIMPID.

FIG. 9 is a view illustrating the sheet resistance in the case where theformation is repeatedly performed by LIMPID.

FIGS. 10A to 10C are views illustrating the steps of forming a thin filmtransistor as an embodiment of the method for doping the semiconductorlayer of the invention. FIG. 10A is a view illustrating the steps untilforming a semiconductor polycrystalline film, FIG. 10B is a viewillustrating the steps until crystallizing a semiconductor film, andFIG. 10C is a view illustrating the steps until adsorbing dopant ion andhydrogen ion.

FIGS. 11A to 11C are views illustrating the steps of forming the thinfilm transistor as the embodiment of the method for doping thesemiconductor layer of the invention, following the steps shown FIG.10C. FIG. 11A is a view illustrating the steps until activating thesemiconductor polycrystalline film, FIG. 11B is a view illustrating thesteps until high concentration doping to the semiconductor thin film,and FIG. 11C is a view illustrating the steps until wiring of the thinfilm transistor.

FIG. 12 is a sectional view illustrating the device structure of thinfilm transistor which is formed according to the method formanufacturing the thin film semiconductor device of the invention.

FIG. 13 is a characteristic view illustrating the relation of PH₃partial pressure and the sheet resistance in the conventional process.

DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENTS

An embodiment of a method for doping a semiconductor layer according tothe invention will be described in detail hereinbelow with reference toFIGS. 1A to 1C. The method for doping the semiconductor layer of theembodiment can form a low concentration impurity diffusion region in asilicon layer formed on a low heat resistant insulating substrate suchas plastic or the like with good controllability.

First, as shown in FIG. 1A, a silicon layer 11 as a semiconductor layeris formed on an insulating substrate 10 (refer to FIG. 1C) and iscrystallized by laser beam irradiation of excimer laser at this time. Inthe surface of the silicon layer 11, a lot of silicon atoms 11 a existin microcrystalline form and dangling bonds 11 b exist in some of thesilicon atoms 11 a.

Next, as shown in FIG. 1B, dopant gas ion is adsorbed to terminate thedangling bonds 11 b on the surface of the silicon layer 11 by plasma ofdopant gas without semiconductor deposition gas. The dopant gas is a gaswhich dilute PH₃ gas containing phosphorus ion 12 p with hydrogen H₂.The phosphorus ion 12 p and hydrogen ion 12 h exist in the plasma gas.Each of the phosphorus ion 12 p and the hydrogen ion 12 h are adsorbedon the surface of the silicon layer 11 to terminate the dangling bonds11 b. When only the phosphorus ion 12 p exist, the phosphorus ion 12 pis adsorbed on most of the dangling bonds 11 b on the surface of thesilicon layer 11. In this case, it is difficult to form the lowconcentration diffusion region even when the partial pressure ofphosphoric gas is reduced with an inert gas (refer to FIG. 13). On theother hand, in the embodiment, the hydrogen ion 12 h controls the ratein which the dangling bonds 11 b are terminated by the phosphorus ion 12p and the concentration of the phosphorus ion 12 p on the surface of thesilicon layer 11 can be reduced during the adsorption.

Subsequently, as shown in FIG. 1C, the excimer laser beam is irradiatedon the surface of the silicon layer 11 formed on the insulatingsubstrate 10 to diffuse the phosphorus ion 12 p adsorbed on the surfaceof the silicon layer 11 into the silicon layer 11. By the excimer laserirradiation, the silicon layer fuses instantly and the adsorbedphosphorus ion 12 p is dissolved in the film. At this time, the hydrogenion 12 h which terminate some of the dangling bonds 11 b are gasifiedand evaporated in an atmosphere of gas as PH₃ gas, whereas thephosphorus ion 12 p is diffused in the silicon layer 11. Thereby the lowconcentration impurity diffusion region is formed. Since the hydrogenion 12 h is adsorbed to terminate the dangling bonds 11 b, thephosphorus ion 12 p is diffused with low concentration and especially,when forming the LDD structure of TFT in the silicon layer 11, the lowconcentration impurity region is formed with good controllability.

In the doping method of the embodiment, the dopant ion gas is diluted byflowing the gas which adsorb the hydrogen ion to control the amount ofthe dopant ion adsorbed on the surface of the semiconductor layer. Inaddition, changing the substrate temperature, controlling the flow rateor partial pressure of gas, further diluting the mixed gas of dopant iongas and hydrogenous gas with an inert gas, and controlling theconditions of the plasma irradiation for adsorption may be applicable tocontrol the amount of the dopant ion. Further, the dopant ion adsorbedon the surface of the semiconductor layer can be removed by etching orthe like and the hydrogen plasma treatment can be added to control theamount of the dopant ion. In addition, a method of removing the dopantion by etching or the like can be performed with combination of a methodof controlling the flow rate or the partial pressure of gas, orcontrolling the substrate temperature.

By mixing the hydrogen gas to the dopant gas, hydrogen passivation isgenerated concurrently with the adsorption of dopant ion on the surfaceof the silicon layer during the plasma treatment. In order to make thedoping ion adsorb to the dangling bonds of the hydrogen-passivatedsilicon, it is necessary to cut the bond of Si and H. In this case,inhibiting the adsorption speed of the doping ion to improve the processcontrollability. In the meanwhile, hydrogen adsorbed on the siliconsurface around room temperature is relatively stable and specifically,when hydrogen and phosphorus are adsorbed together, it forms P—Si—Hcomplex and the dissociation energy of hydrogen increases, thereby Si—Hbond is stable. However, this Si—H bond is easily broken when thesubstrate temperature becomes about 100° C. In order to form the lowtemperature impurity diffusion region of the LDD structure, it isnecessary to reduce the adsorption amount of phosphorus. This can beachieved by irradiating plasma gas containing doping gas and a largeamount of hydrogen around room temperature. On the other hand, in orderto form the high concentration impurity diffusion region as the sourceand drain regions, it is necessary to increase the adsorption amount ofphosphorus. This can be achieved by decreasing the concentration ofhydrogen to raise the substrate temperature and irradiating plasma tothe raised substrate. Thereby, the resistance of thin film after theexcimer laser anneal can be controlled with good accuracy.

Further, the adsorbed dopant ion is activated by irradiation of energybeam like excimer laser as shown in FIG. 1C, and the activation of thedopant ion and the recrystallization of the semiconductor layer on thesubstrate can be performed simultaneously. An example of the energy beamto be used for activation of the dopant ion or the recrystallization ofthe semiconductor layer on the substrate is excimer laser such as ArFexcimer laser, XeF excimer laser, XeCl excimer laser, KrF excimer laserand the like. The dopant ion may be activated by using the heatingmethods such as lamp anneal, furnace anneal and the like in addition tothe energy beam such as excimer laser.

When controlling the conditions of the plasma irradiation during thedopant ion adsorption, plasma can be generated from the mixed gas ofdoping gas and hydrogen gas or mixed gas of inert gas and hydrogen gasto irradiate plasma. In addition to this, hydrogen plasma irradiationmay be performed before or after the plasma irradiation with a gascontaining doping gas. In general, the higher the concentration ofhydrogen gas which dilutes the doping gas is, the lower theconcentration of impurity diffusion is, and the lower the temperature ofthe surface of the semiconductor layer is, the lower the concentrationof impurity diffusion is. The conditions of the plasma irradiationinclude flow rate and pressure of mixed gas containing dopant gas andhydrogen gas, substrate temperature, irradiation time, RF power or thelike. Controlling these conditions alone or in combination enables toform the low concentration impurity diffusion region with goodreproducibility.

The inventors of the present invention have experimented various methodsfor controlling the amount of dopant ion adsorbed on the surface of thesemiconductor layer by controlling the diluted gas, the plasmairradiation time, the substrate temperature, and the hydrogen plasmatreatment. Here, data obtained form the results of the experiments willbe described with reference to FIGS. 2 to 6.

Phosphine (PH₃) was used as the dopant gas and was diluted with ahydrogen gas (H₂) and an argon gas (Ar) to have the concentration ofabout 1% and the plasma was irradiated to a nonsingle crystal siliconfilm with a thickness of 70 nm with the gas. The sheet resistance ρ_(s)was measured. The results are shown in FIG. 2. The mixed gas of argongas and hydrogen gas as the inert gas was mixed by varying the hydrogenpartial pressure from 0% (argon partial pressure of 100%) to 100% (argonpartial pressure of 0%). The horizontal axis indicates the mixture ratioof the mixed gas of dopant gas containing hydrogen and argon (partialpressure ratio of argon gas Ar) and the vertical axis indicates thesheet resistance ρ_(s). The gas containing phosphine (PH₃) and about 99%of hydrogen gas (H₂) is 9.2 Pa (69 mTorr) and the partial pressure ofargon gas in the mixed gas of argon and hydrogen is 54.1·X Pa (406·XmTorr) (X is numeric value (%)). The partial pressure of hydrogen gas inthe mixed gas of argon and hydrogen is 54.1·(100−X) Pa (406·(100−X)mTorr) (X is numeric value (%)).

As seen in FIG. 2, high resistance of about 5×10³ to 6×10³ Ω/cm² can beachieved particularly in the region where the partial pressure of argongas is low, that is, the partial pressure of hydrogen gas is high andthis is the effect of the dilution of dopant gas with the hydrogen gas.Namely, the existence of hydrogen gas allows the adsorption of thehydrogen gas on the thin film semiconductor film with high ratio andreduction of phosphorus concentration to achieve the high resistance.

FIG. 3 shows a change in the concentration of phosphorus ion to theenergy density (mJ/cm²) of the excimer laser. The points indicated byblack squares have the argon partial pressure of 100% and the pointsindicated by black circles have the hydrogen partial pressure of 100%.The lower the energy density of the excimer laser is, the higher theconcentration of phosphorus ion is. The case where the argon partialpressure is 100% has higher concentration of phosphorus ion compared tothe case where the hydrogen partial pressure is 100%. When forming thelow concentration impurity diffusion region, the hydrogen partialpressure higher than the argon partial pressure is preferable.

The inventors of the present invention have experimented the relation ofplasma irradiation time and the sheet resistance as shown in FIG. 4.This experiment was carried out to improve the adsorption of phosphoruscontrolled by the plasma irradiation time and the activation by theexcimer laser anneal of the 70 nm-thick nonsingle crystal silicon film.The horizontal axis indicates plasma irradiation time (second) and thevertical axis indicates the sheet resistance ρ_(s) (Ω/□). As seen inFIG. 4, the longer the plasma irradiation time is, the lower the sheetresistance ρ_(s) is. This reveals that the high resistance of theimpurity diffusion region is achieved by shortening the plasmairradiation time. The decrease of the sheet resistance ρ_(s)substantially indicates exponential change and within the short timeafter starting the irradiation, the resistance decreases significantly.In this experiment, the conditions of the plasma irradiation other thanirradiation time were as follows: the flow rate of the mixed gas ofphosphine (PH₃) and 99% of hydrogen (H₂) was 10 sccm, the flow rate ofargon gas was 50 sccm, the pressure was 63.3 Pa (475 mTorr), thesubstrate temperature was 130° C. and the RF power was 20 W. The XeClexcimer laser (wavelength of 308 nm) was used as the excimer laser interms of the condition of the excimer laser anneal for activation andthe XeCl excimer laser was irradiated sequentially with 98% overlap.

The experiment reveals that as a method for controlling the amount ofdopant ion adsorbed on the surface of the semiconductor layer, thecontrol of the substrate temperature during the plasma irradiation isalso effective. FIG. 5 shows the results of the measurement of a changein the sheet resistance ρ_(s) of the nonsingle crystal silicon layerwith a film thickness of 70 nm by changing the substrate temperature(K). The horizontal axis indicates the substrate temperature (K) and thevertical axis indicates the sheet resistance ρ_(s) (Ω/□). As seen inFIG. 5, the higher the substrate temperature during the plasmairradiation is, the lower the sheet resistance ρ_(s) is. The highresistance of the impurity diffusion region can be achieved byincreasing the substrate temperature during the plasma irradiation. Inthis experiment, the conditions of the plasma irradiation other than thesubstrate temperature were as follows: the flow rate of the mixed gas ofphosphine (PH₃) and 99% of hydrogen (H₂) was 10 sccm, the flow rate ofargon gas was 50 sccm, the pressure was 63.3 Pa (475 mTorr), the RFpower was 20 W and the plasma irradiation time was 1 minute. The XeClexcimer laser (wavelength of 308 nm) was used as the excimer laser interms of the condition of the excimer laser anneal for activation andthe XeCl excimer laser was irradiated sequentially with 98% overlap.

Furthermore, the hydrogen plasma treatment can be added to control theamount of the dopant ion adsorbed on the surface of the semiconductorlayer. As shown in FIG. 6, after performing the plasma irradiation byuse of the doping gas, the plasma irradiation by use of the hydrogen gaswas performed to substitute the hydrogen ion for the phosphorus ionalready adsorbed on the surface of the semiconductor thin film tocontrol the amount of the adsorbed dopant ion. FIG. 6 shows thedependency of the hydrogen plasma irradiation of the sheet resistanceρ_(s) (Ω/□). The horizontal axis indicates the irradiation time ofhydrogen plasma (second) and the vertical axis indicates the sheetresistance ρ_(s) (Ω/□). The longer the time of the hydrogen plasmatreatment is, the higher the sheet resistance ρ_(s) is. Therefore, theimpurity diffusion region having a desired sheet resistance (Ω/□) can beformed by controlling the time of the hydrogen plasma treatment. In thisexperiment, the nonsingle crystal silicon film having a film thicknessof 40 nm was used as the semiconductor film and the conditions of theplasma irradiation of doping gas were as follows: the flow rate of themixed gas of phosphine (PH₃) and 99% of hydrogen (H₂) was 10 sccm, theflow rate of argon gas was 50 sccm, the pressure was 63.3 Pa (475mTorr), the substrate temperature was 130° C., the RF power was 20 W andthe plasma irradiation time was 1 minute. The conditions of hydrogenplasma irradiation were as follows: the flow rate of hydrogen gas (H₂)was 50 sccm, the pressure was 26.7 Pa (200 mTorr) and the RF power was20 W. The XeCl excimer laser (wavelength of 308 nm) was used as theexcimer laser in terms of the condition of the excimer laser anneal foractivation and the XeCl excimer laser was irradiated sequentially with98% overlap.

As apparent from the results of the above experiments, in the method fordoping the semiconductor layer of the embodiment, the amount of thedopant ion adsorbed on the surface of the semiconductor layer iscontrolled. Specifically, this is controlled by diluting the dopant iongas by flowing the gas which adsorb the hydrogen ion, changing thesubstrate temperature, controlling the flow rate or partial pressure ofgas, further diluting the mixed gas of dopant ion gas and hydrogenousgas with the inert gas, and controlling the conditions of plasmairradiation such as substrate temperature or plasma irradiation time foradsorption. The amount of the dopant ion adsorbed on the surface of thesemiconductor layer can be controlled by removing the dopant ion alreadyadsorbed on the surface of the semiconductor layer by etching or thelike, or adding the hydrogen plasma treatment.

Here, the LIMPID using the excimer laser will be described. Basically,this method is a method that dissolving impurity ion into the film byirradiation of energy beam like the excimer laser after ionizing thedopant gas and adsorbing the dopant ion on the surface of thesemiconductor thin film. This specifically attracts the attention toachieve the process in a low temperature. In the method for doping thesemiconductor layer of the embodiment, the amount of the dopant ionadsorbed on the surface of the semiconductor layer is controlled in astep of adsorbing the dopant, however, after the step, doping isperformed with the same method as the LIMPID using the excimer laser.

The LIMPID is excellent in the uniformity of doping in the surface andis especially suitable for diffusing the low concentration impurity.FIG. 7 shows the film thickness distribution when forming the thin filmsuch as silicon with CVD apparatus and the dots indicated by number 1 tonumber 9 in the drawing is measurement point of the sheet resistance.The each distribution of the sheet resistance indicated by number 1 tonumber 9 is shown in FIG. 8. The distribution in the wafer surface wasmeasured for a first gas with the partial pressure of the mixed gas ofphosphine and hydrogen (PH₃/H₂) of 5.2 Pa (39 mTorr), the partialpressure of hydrogen gas of 29 Pa (218 mTorr) and the partial pressureof argon gas of 29.1 (218 mTorr), and for a second gas with the partialpressure of the mixed gas of phosphine and hydrogen (PH₃/H₂) of 9.5 Pa(71 mTorr) and the partial pressure of argon gas of 53.9 (404 mTorr). Asseen in FIG. 8, the sheet resistance using the LIMPID is substantiallyuniform in the surface, thereby obtaining the good reproducibility.

FIG. 9 shows a reproducibility of LIMPID and the fluctuation of thesheet resistance which was measured in the state where the amorphoussilicon film was plated and the laser anneal was applied. In FIG. 9, thedistribution of the upper part is a data of a first conditions: the flowrate of the mixed gas of phosphine and hydrogen (PH₃/H₂) is 9 sccm, theflow rate of the hydrogen gas is 92 sccm and the flow rate of the argongas is 50 sccm, while the distribution of the lower part is a data of asecond conditions: the flow rate of the mixed gas of phosphine andhydrogen (PH₃/H₂) is 9 sccm and the flow rate of the argon gas is 50sccm. The data indicated by the triangles is the data reprocessed after6 days from the data indicated by the squares. The data indicated by thetriangles and the squares overlapped well and this exhibits that LIMPIDcan form the device excellent in the reproducibility.

The doping process of the invention is especially suitable for theprocess with a low temperature. For example, the plastic can be used asa material of the substrate. Here, the process using the plasticsubstrate will be described with reference to FIGS. 10A to 11C. In theembodiment, as an example, a p-channel type thin film transistor isformed on an insulating substrate and then it is used for an activedevice substrate of an active matrix display to fabricate a thin filmsemiconductor device. This can be used when forming an n-type channeltype thin film transistor as well.

First, in FIG. 10A, an organic polymeric material, so-called plasticmaterial is used to form an insulating substrate 15. The plasticmaterial used herein may include polyesters such as a polyethyleneterephthalate, polyethylenenaphthalate and polycarbonate, polyolefinessuch as polypropylene, polyphenylene sulfides such as polyphenylenesulfide, polyamides, aromatic polyamides, polyether ketones, polyimides,acrylic resins, PMMA (Polymethyl Methacrylate), and the like. Inparticular, it is preferred to use a general-purpose plastic materialsuch as polyethylene terephthalate, acetate, polyphenylene sulfide,polycarbonate, polyether sulfone, polystyrene, nylon, polypropylene,polyvinyl chloride, the acrylic resins, PMMA, and the like. Moreover,when using a film type as the insulating substrate 15, it is preferredthat the film is extended by biaxial stretching in the light ofmechanical stability and strength. Furthermore, a barrier layer 16 suchas a silicon oxide film can be formed on the back side of the substratefor suppressing the hygroscopic property of the plastic plate.Therefore, deformation of the insulating substrate 15 can be suppressedin exposure to atmospheric pressure after being taken out of a vacuumapparatus and in the subsequent processes. As shown in FIG. 10A, thebarrier layer 16 of silicon oxide can be formed on the front side of theinsulating substrate 15 made of the plastic, which is more effective.

Moreover, it is desirable to pre-form a thermal buffer layer 20 on theinsulating substrate 15. As the thermal buffer layer 20, it is preferredto form an inorganic material film such as a SiO₂ film or a SiN_(x) filmwith a thickness of about 100 to 500 nm. At this time, it is moreeffective that a multilayer structure is constructed by forming anotherbuffer layer 17, of an organic polymeric material such as an acrylicresin with a thermal softening point lower than that of the substrate,on the buffer layer 20, for the purpose of preventing film separation atthe time of energy beam irradiation due to the difference in the thermalexpansion between the organic polymeric material of the insulatingsubstrate 15 and the inorganic material.

Then, an amorphous semiconductor thin film 21 which serves as an activelayer of the transistor is formed on the upper surface of the insulatingsubstrate 15 on which such heat resistant buffer layers 17 and 20 arealready formed. In the embodiment, the amorphous semiconductor thin filmis deposited to a thickness of about 20 to 100 nm using a sputteringapparatus as a deposition apparatus, at the substrate temperature set to200° C. or below, preferably 150° C. or below, where the insulatingsubstrate 15 may not be damaged. Then, a semiconductor polycrystal film22 is formed by irradiating an energy beam, for example, an excimerlaser, onto the insulating substrate 15 and crystallizing the amorphoussemiconductor thin film 21. Also in the crystallization by theirradiation of the excimer laser, optimization of both time and theirradiation energy of the laser is required so that the temperature ofthe plastic of the insulating substrate 15 is kept at 200° C. or below,preferably 150° C. or below, where the plastic substrate may not bedamaged. At this time, it is desirable to irradiate the laser beamrepeatedly, for example, while the line-shaped laser beam, which energydensity is set at about 300 mJ/cm², is scanning the substrate so thatthe irradiated parts partially overlap. The size of the line-shapedlaser beam is, for example, 120 mm in longitudinal size and 0.5 mm inwidth. The laser beam is irradiated along the width direction withpartially overlapping the irradiated parts, where the amount of theoverlapping part (the overlapping amount) is set to 98%, for example.Then, a SiO₂ film used as a gate insulating film 23 is formed on theupper surface of the semiconductor polycrystal film 22 by reactivesputtering, as shown in FIG. 10B. Alternatively, another SiO₂ film maybe subsequently formed on a SiN_(x) film which is formed on the SiO₂film and the stacked film may be used as the gate insulating film 23.

Then, a gate electrode 24 is formed on the gate insulating film 23. Thegate electrode 24 is formed by depositing a metal film such as Al, Mo,Ta, Ti, Cr or the like, a polysilicon film into which high concentrationimpurities are doped, a laminated film of high concentration dopedpolysilicon and a metal, or an alloy film of the above mentionedmaterials and being patterned to a predetermined shape to form the gateelectrode. Next, by using the gate electrode 24 as a mask, theinsulating film 23 is patterned to the island shape, thereby the gateinsulating film 23 of the thin film transistor is formed.

Subsequently, the LDD region is formed. As shown in FIG. 10C, the plasmais generated by using the mixed gas of doping gas and hydrogen gas orthe mixed gas of inert gas and hydrogen gas, and make the doping ion andthe hydrogen ion adsorb on the surface of the semiconductor polycrystalfilm 22. At this time, when the temperature of the surface of thesemiconductor polycrystal film 22 is close to room temperature and theconcentration of the hydrogen in the diluted gas is low, the doseincreases. This increase becomes close to a certain concentration astime advances. This is because phosphorus is not easily adsorbed on theregion where phosphorus was already adsorbed and the adsorptiondecreases as the coverage of phosphorus increases. In the embodiment,the substrate temperature is set to room temperature and the phosphorusion is adsorbed on the surface by irradiating plasma for about 1 minuteat 66.7 Pa (500 mTorr) with the source gas mixing PH₃ gas (diluting withH₂ to be the PH₃ concentration of 1%) of 9 sccm and hydrogen diluted gasof 100 sccm for doping and the RF power of 20 W. In the adsorption step,the amount of dopant ion adsorbed on the surface of the semiconductorpolycrystal film 22 is controlled by controlling the flow rate of thediluted gas. However, it may be controlled by changing the substratetemperature, further diluting the mixed gas of dopant ion gas andhydrogenous gas with inert gas, controlling the conditions of plasmairradiation for adsorption such as substrate temperature or plasmairradiation time. In addition, it is possible to remove the dopant iononce adsorbed on the surface of the semiconductor polycrystal film 22 byetching or the like, and the hydrogen plasma treatment may be added tocontrol the amount of the dopant ion adsorbed on the surface of thesemiconductor polycrystal film 22.

After that, as shown in FIG. 11A, the energy beam is again irradiated todissolve the dopant adsorbed on the surface of the semiconductorpolycrystal film 22 and to activate it. At this time, as describedabove, the XeCl excimer laser (wavelength of 308 nm) is used as theenergy beam and a laser having higher energy than the one used in thecrystallization (microcrystallization) of the semiconductor thin film 21is desirable. In the embodiment, the energy density of the laser beam isset to about 300 mJ/cm². Thereby, the region, which is not masked by thegate electrode 24, in the semiconductor polycrystal film is activatedwith low dose and a low concentration impurity region 22L is formed.

Then, sidewalls 25 are formed on the gate electrode 24. On the entiresurface including the gate electrode 24, SiO₂ film is deposited by, forexample, PE-CVD (Plasma Enhanced-Chemical Vapor Deposition). After that,as shown in FIG. 11B, the SiO₂ film is removed using the gate electrode24 as a stopper to leave the SiO₂ film on the sides of the gateelectrode 24 and the gate insulating film 23 to make the sidewalls 25 byanisotropic etching (for example, reactive ion etching (RIE)) or thelike. The plasma is generated by using the mixed gas of doping gas andthe hydrogen gas or the mixed gas of inert gas and hydrogen gas to beagain adsorbed on the surface of the semiconductor polycrystal filmusing the gate electrode 24 and the sidewalls 25 as a mask. Then, heavydoping for forming a high concentration impurity region 22H is performedby again irradiating the energy beam such as XeCl excimer laser(wavelength of 308 nm) to dissolve the dopant adsorbed on the surface ofthe semiconductor polycrystal film (the low concentration impurityregion 22L) and to activate it. In the embodiment, the substratetemperature is set to 120° C. and the phosphorus ion is adsorbed on thesurface by irradiating plasma for about 1 minute at 66.7 Pa (500 mTorr)with the source gas mixing PH₃ gas (diluting with H₂ to be the PH₃concentration of 1%) of 9 sccm and Ar diluted gas of 50 sccm for dopingand the RF power of 20 W. The diluted gas can include other inert gassuch as He gas, Ne gas or the like. The energy beam is irradiated asdescribed above and the energy density of the laser beam is set at about310 mJ/cm² in the example. Therefore, the region where is not maskedwith the gate electrode 24 and the sidewalls 25 in the semiconductorpolycrystal film is activated at high dose. The dopant for forming thehigh concentration impurity region 22H and the low concentrationimpurity region 22L may be the same or different dopant than phosphorus.

After activating the high concentration impurity region 22H, it isselectively removed except the regions become the source and drainregions by etching. Then, an interlayer insulating film 28 is formed andthe required contact hole is formed. On the insulating film 28, analuminum film is formed with a thickness of about 1 μm and patterned toa predetermined shape to form a wiring electrode 27 as shown in FIG. 1C.The wiring electrode 27 is connected to the source and drain regionsthrough the contact hole. Next, the SiO₂ film is formed with a thicknessof about 400 nm to form a passivation film. The passivation film coversthe thin film transistor and the wiring electrode 27. Then, if required,the so-called hydrogenation treating is carried out by heating thesubstrate in the region of a heat resistant temperature of thesubstrate, and diffusing the hydrogen atoms contained in the interlayerinsulating film 28 into the semiconductor polycrystal film by using thepassivation film as a cap film.

FIG. 12 is a sectional view of a device in the case of composing anactive matrix display. Barrier layers 41 made of, for example, oxidizedsilicon are formed on both sides of an insulating substrate 40 made fromplastic, and a heat resistant buffer layers 42 and 43 of the insulatingsubstrate 40 of plastic are laminated and a semiconductor thin filmcomprising a high concentration impurity region 44H and a lowconcentration impurity region 44L are formed thereon. In particular, thelow concentration impurity region 44L controls the amount of dopant ionadsorbed on the surface of the semiconductor thin film by controllingthe flow rate of the dilute gas. The low concentration impurity region44L may be controlled to be a certain low concentration by changing thesubstrate temperature, further diluting the mixed gas of the dopant iongas and the hydrogenous gas with the inert gas, controlling theconditions of the plasma irradiation for adsorption such as substratetemperature or plasma irradiation time.

On the channel region between a pair of low concentration impurityregions 44L, a gate electrode 47 is formed with a silicon oxide film 46in between. A wiring electrode layer 48 is connected to the source anddrain regions (high concentration impurity region 44H) of the thin filmtransistor through the contact hole provided in an interlayer insulatingfilm 45. A transparent conductive film made of ITO (Indium Tin Oxide) orthe like is formed on a surface of a passivation film 49 and patternedinto a predetermined shape to form a pixel electrode 50. The pixelelectrode 50 is pre-connected to the wiring electrode layer 48 and tothe source and drain regions of the thin film transistor trough thecontact hole preformed in the passivation film 49 and the interlayerinsulating film 45. When composing the active matrix liquid crystaldisplay using the thin film semiconductor device as the active devicesubstrate, another insulating substrate on which preformed the facingelectrode is connected to the insulating substrate 40 with a certainspace and electro-optic substance such as liquid crystal or the like isplaced to the space.

In the above described embodiment, the method for doping thesemiconductor layer and the method for manufacturing the thin filmsemiconductor device using the same are described. This is the method toimprove the resistance of the semiconductor layer with goodcontrollability, so it may be applied for controlling the resistance ofthe resistance layer taking advantage of the step of controlling theamount of the dopant ion adsorbed on the surface of the semiconductorthin film.

As described above, according to the method for doping the semiconductorlayer of the invention, the low concentration impurity diffusion regioncan be formed with good controllability even in case of using the lowheat resistant substrate and particularly, the low concentrationimpurity region of the LDD structure in the thin film transistor can beformed with good controllably.

Obviously many modifications and variations of the present invention arepossible in the light of the above teachings. It is therefore to beunderstood that within the scope of the appended claims, the inventionmay be practiced otherwise than as specifically described.

1-20. (canceled)
 21. A thin film semiconductor device, comprising: athin film semiconductor layer formed on an insulating substrate; sourceand drain regions formed in the semiconductor layer; and a pair of lowconcentration impurity diffusion regions are formed in the channel sidesof the source and drain regions by controlling the amount of the dopantion adsorbed on the surface of the semiconductor layer and followed byactivating the adsorbed dopant ion.